Cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (communication units). The service coverage areas of adjacent cells may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system.
A cell's radio coverage is provided by a base transceiver station (BTS). Each BTS may contain one or more transceivers (TRX) which can simultaneously receive on one frequency and transmit on another. Communication between a BTS and a mobile communication unit (or mobile station) (MS) typically occurs using a portion of a pair of frequencies (transmit and receive) temporarily assigned in support of the communication transaction at the BTS.
The pair of frequencies assigned for use at the remote site are typically referred to as a radio channel. Downlink transmissions (from BTS to MS) on the radio channel occur on a first frequency of the pair of frequencies. Uplink transmissions (from MS to BTS) on the radio channel occurs on the second frequency of the pair of frequencies.
The GSM system is a TDM/TDMA system providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assigned to a BTS, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots. Additional, secondary radio channels assigned to the same cell can provide a full complement of eight full rate traffic users (in the 8 TDM slots) per radio channel, since the control channel within the primary radio channel can control allocation of communication resources on secondary radio channels.
Transmissions (control or speech and/or data traffic) from a BTS to an MS, on the downlink, occupy a first TDM slot (downlink slot) on a first frequency of a radio channel and transmissions from a communication unit to a BTS, on the uplink, occupy a second TDM slot (uplink slot) on the second frequency of the radio channel. The uplink slot on the second frequency is displaced in time three TDM slot positions following the downlink slot on the first frequency. The uplink slot on the second frequency is offset 45 MHz lower in frequency than the downlink. The uplink slot and downlink slot (together providing a two-way signal path for a single user) may be referred to as a "communication resource", allocated by the BTS to an MS for exchanging signals. The term "communication resource" also typically includes an associated signalling channel, as for example the GSM specified slow associated control channel used with traffic channels.
Exchanges of paging and setup control information between MSs and BTSs typically occurs on the common control channel (CCCH) which occupies at least one slot of a primary channel of the BTS. The CCCH may logically share a slot on the primary channel with a broadcast control channel (BCCH). Transmitted by the BTS on the BCCH are distinctive identification signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. BCCH information allows an MS to differentiate between primary and non-primary channels.
Upon activation, an MS scans a set of frequencies in search of BCCH identification signals transmitted from proximate BTSs. Upon detecting a BCCH identification signal, the communication unit measures a signal quality factor (such as signal strength) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the set, the MS generally selects the BTS providing the largest relative signal quality factor as a serving BTS. Upon identifying, and locking onto a suitably strong signal (and registering if necessary) the communication unit monitors the selected BCCH for incoming calls. Should the communication unit desire to initiate a call, an access request may be transmitted using the CCCH of the serving BTS.
During normal operation (including during active calls), the MS monitors for, identifies, and measures primary channels of nearby BTSs. If involved in an active call, the MS relays measurement information back to the base site on an associated signaling channel. Through such a process, it is possible for the MS to maintain an association with the most appropriate (proximate) BTS. The process may entail an autonomous switching by the MS to a different BTS, causing perhaps a re-registration by the MS with the system indicating that such a switch has occurred. Alternatively, during an active communication exchange, the MS may be commanded by the system to handover to a more appropriate BTS.
The GSM system specification allows for frequency hopping of the radio channel. Under frequency hopping in GSM, the exchange of communicated signals between the BSS and MS may occur on an indexed frequency which may index to a different RF channel after each TDM frame. As is known, the combination of frequency hopping, in conjunction with error correction coding techniques, results in a significant improvement in reception quality and improved signal robustness due to averaged radio channel fading and interference noise.
Channel fading, within cellular communication system in general and as is known in the art, may arise from signal barriers (such as hills or buildings) and/or from multipath signal propagation (Rayleigh fading). Interference noise on a radio channel may arise from the simultaneous operation of MSs on the same or adjacent channels, generating mutually interfering signals.
Simultaneous use of communication channels within a communication system may often be required by a limited frequency spectrum and by the number of users within the system. Simultaneous use of a radio channel may be possible within the system through the maintenance of minimum separation distances between simultaneous channel users. Maintenance of minimum distances may reduce mutual interference within a communication system below a maximum threshold level required by the system.
To maximize the capacity of a cellular radiotelephone system within a given geographic area, operating frequencies must be reused among the cells in such a manner that mutual interference does not exceed the maximum threshold level. Mutual interference, within such systems, is maintained below the maximum threshold level through the use of a reuse pattern.
Mutual interference is typically considered as arising from two sources. One source is co-channel interference and occurs when two cellular radiotelephones transmit on the same frequency. The second source is called adjacent channel interference. Adjacent channel interference typically arises when two radiotelephones operating on adjacent channels attempt to exchange a signal with a cellular base site from different areas of the cell (one near the base site the other near the fringe of the cell). In such a case the magnitude of a received signal from the radiotelephone near the base site "blinds" the base to a signal from the fringe.
The reuse pattern controls co-channel interference by maintaining a minimum distance between reusing base sites. The minimum distance under the reuse pattern is typically specified in terms of cell radius (see Graziano, U.S. Pat. No. 4,128,740).
The reuse pattern controls adjacent channel interference by sequentially assigning frequencies within the cellular system so as to avoid assignment of adjacent channels to the same, or adjacent, cells. Limitations on the use of adjacent frequencies in adjacent cells arises because, by the nature of cells in cellular telephony, a radiotelephone in a first cell may present a stronger signal to a base site in a second cell than another radiotelephone operating from within the second cell.
While prior art reuse methods have worked well considerable spectral efficiency may be lost due to prohibitions on intra-cell adjacent channel utilization and due to multi-cell reuse patterns. A need exists for a radio channel assignment method that accommodates use of intra-cell adjacent channels and that allows for reduced reuse distances (near-cell) of radio channels within a frequency spectrum. Such an assignment method should accommodate frequency hopping methodologies and channels of different bandwidths.